MODES OF ACTION. 225 
organism, or to exceptional pathogenic power acquired by some one 
of the common bacteria which infest suppurating wounds, cannot be 
determined in the absence of exact experiments by modern methods. 
But the latter view has seemed to the writer the most probably cor- 
rect. There are many facts which go to show that pathogenic viru- 
lence may be increased by cultivation in animal fluids, and where 
wounded men are brought together under unfavorable sanitary con- 
ditions, as has been the case where hospital gangrene has made its 
appearance, it may be that some common saprophyte acquires the 
power of invading the exposed tissues instead of simply feeding upon 
the secretions which bathe its surface. 
Koch has described a progressive tissue necrosis in mice, due to a 
streptococcus, which he first obtained by inoculating a mouse in the 
ear with putrid material. The morbid process is entirely local and 
rapidly progressive, causing a fatal termination in about three days, 
without invasion of the blood. 
In diphtheritic inflammations of mucous membranes we have 
a local invasion of the tissues and a characteristic plastic exudation. 
In true diphtheria the local inflammation and necrotic changes in 
the invaded tissues are not sufficient to account for the serious gen- 
eral symptoms, and we now have experimental evidence that the 
diphtheria bacillus produces a very potent toxic substance to which 
these symptoms are no doubt largely due. The diphtheria bacillus 
of Léffler appears to be the cause of the fatal malady which goes 
by this name, but undoubtedly other microérganisms may be con- 
cerned in the formation of diphtheritic false membranes. In cer- 
tain forms of diphtheria, and especially when it occurs as a com- 
plication of scarlet fever, measles, and other diseases, the Klebs- 
Léffler bacillus is absent, and a streptococcus, which appears to be 
identical with Streptococcus pyogenes, is found in considerable num- 
bers and is probably the cause of the diphtheritic inflammation. 
An epidemic of diphtheria occurring among calves was studied by 
Loffler, and is ascribed by him to his Bacillus diphtheriz vitulo- 
yum. The same bacteriologist has shown that the diphtheria of 
chickens and of pigeons is due to a specific bacillus which differs 
from that found in human diphtheria, and which he calls Bacillus 
diphtheria columbrarum. 
Prof. Welch has studied the histological lesions produced by 
filtered cultures of the diphtheria bacillus. Cultures in glycerin- 
bouillon, several weeks old, were filtered through porcelain, and the 
sterile filtrate was injected beneath the skin of guinea-pigs. One 
cubic centimetre of this filtrate was injected into a guinea-pig on 
the 10th of December, and two cubic centimetres more on the 
14th of the same month. The animal succumbed at the end of 
15 
